Deactivation of spent silicon powder

ABSTRACT

Residual silicone from direct process manufacture of chlorosilanes is stabilized for transportation and disposal.

This application is a continuation of application Ser. No. 041,184,filed 4/22/87, now abandoned.

The present invention relates to a process for treating silicon andresidual silicon powder. More particularly, the present inventionrelates to a process for forming stable pellets of silicon and residualsilicon powder.

BACKGROUND OF THE INVENTION

The present commercial method for manufacturing organohalosilanes iswell known and is described in U.S. Pat. No. 2,380,995, issued toRochow. This method, known as the "direct process" , involves the directreaction of an organo-halide, such as methyl chloride, with siliconparticles in order to produce organochlorosilane. Intermixed with theparticles of silicon are particles of copper, forming a reactive mass ora reactive contact mass. In commercial practice, the direct process isgenerally carried out in one of three types of equipment: the stirredbed type of reactor as described in Sellers, U.S. Pat. No. 2,449,821;the fluidized bed reactor, as described in Reed, at al., U.S. Pat. No.2,389,931; or the rotary kiln.

At some point in the production of the organochlorosilanes by the directprocess, the reactive mass or reactive contact mass containing siliconparticles becomes less reactive. Thus, spent or less reactive siliconparticles are removed from the reactor and new silicon particlesinserted so that a continuous reaction may continue unabated or so thata batch reaction may be restarted. The spent contact mass is generallyreferred to as residual silicon, residual silicon powder, residualsilicon-containing contact mass or residual contact mass and these termsare used interchangeably herein.

A problem with the residual silicon is that it is highly reactive andexothermic upon exposure to moisture. Thus, disposal or transportationof the residual silicon is complicated by reaction with atmosphericmoisture and other sources of water to produce heat, hydrochloric acid,hydrogen gas and other products.

It is an object of the present invention to stabilize the residualsilicon to permit transportation and disposal.

It is another object of the present invention to produce stable pelletsof residual silicon for transportation and disposal.

DETAILED DESCRIPTION OF THE INVENTION

Briefly, according to the present invention, there is provided a processfor stabilizing residual silicon comprising:

(a) pelletizing the residual silicon and

(b) impregnating the pellets with an organic binder.

The residual silicon is a finely divided powder having an averageparticle diameter in the range of from about less than 1 to about 200microns. It is contaminated with a variety of substances with silicon onan elemental basis comprising about 50 to about 85% by weight. Othersignificant elements present include, of course, chlorine, hydrogen andoxygen in addition to copper, carbon, iron, aluminum, zinc, calcium,tin, lead titanium, manganese and others.

The residual silicon may be pelletized by pressure compaction or bymixer agglomeration. Pressure compactor includes, for example, a pistonor molding press, a tableting press, a roll-type press, a pellet mill ora screw extruder. The preferred method of pelletizing is by mixeragglomeration, which may be accomplished on, for example, an inclinedpan or disk, i.e. a rotary-drum agglomerator, a paddle mixer, i.e. a pugmill, or a flow mixer. The preferred method of mixer agglomeration is byuse of an inclined pan.

As a general proposition, the larger and denser the pellet, the lesssurface area available for reaction. Preferred pellet diameters rangefrom about 1/16 inch to about 3/4 inch. Pellets that are too large aredifficult to handle and may develop hot reaction points within thepellet.

The organic binder is applied to impregnate the pellet either at thetime of pelletization or in a subsequent step. The binder may be at 100%liquid organic binder, where it is of sufficiently low viscosity topenetrate the pellet, or it may be applied in solution or emulsion.

The organic binder should be such that it will penetrate the pellet andimprove cohesiveness therein. Preferred organic binders include, forexample, glucose, glues, gums, asphalt, resins, starch, waxes with apreferred binder being lining. Although organic polymers can beutilized, such as, acrylic polymers or epoxy polymers, they are notpreferred due to expense.

The preferred binder is lining, due both to its availability at low costand to its easy water solubility. Suitable lining includes sulfitelignins from the sulfite pulping process (e.g. calcium or sodiumlignosulfate), alkali lignins from kraft and soda pulping processes,furafil from cereal waste, hydrolysis lignin from glucose production andthe like. Persons skilled in the art will easily recognize suitablelignin.

The solvent or emulsion carrier may be either an organic solvent orwater. Where water is utilized, it should be used in such amounts thatit will act not only as a solvent or emulsion carrier but as a heattransfer fluid to quench the heat of reaction. Such heat of reactionresults from the reactivity of the water with the residual silicon. Inspite of its reactivity, water is preferred because of solvent emissionproblems with organic solvents.

The organic binder is applied to the pellets in sufficient amount thatthe cohesiveness of the pellet as well as its stability is improved.Thus, not only will the binder act as an adhesive to prevent the pelletfrom disintegrating, but it will also serve to coat the individualparticles of the pellet and prevent their uncontrolled contact withmoisture. It is desirable that impregnation of the pellet be sufficientthat the pellet will not exceed about 50° C. upon exposure to moistureand atmospheric oxygen. Generally, a water solution or emulsion bath oforganic binder containing from about 25% to about 75% by weight solidswill provide the desired impregnation.

The resultant pellets may be air dried or oven dried to remove solventand to cure or set-up the binder material. Of course, it is preferableto avoid heating costs where possible so air drying is preferred.However, oven drying may be desirable in some instances to produce amore durable pellet.

The above described method will produce pellets that are safe fortransportation to ultimate disposal sites. Persons skilled in the artcan easily imagine suitable variations.

The following examples are illustrative of aspects of the presentinvention and are not in any way intended to limit the scope of theinvention.

EXAMPLES

Residual silicon powder having a bulk density of 68 lb/ft³ waspelletized in a 3-ring drum pelletizer at a 30° angle to produce 1/4inch diameter pellets. Water, organic solids, and organic solutions inwater as shown below were employed as binders. Water and organicsolutions in water were sprayed through the pelletizer as necessary toform the green pellets and cool the reaction. Green pellets were airdried and oven dried at 250° as shown and subjected to drop and crushtests. The figure shown for the drop test was the number of times thatpellets survived drops from 18 inches before breakage. The figure shownfor the crush test is in pounds and is the load that must be appliedacross the diameter of a pellet before it fragments or crumbles.

    __________________________________________________________________________    FIG. 1                                                                                       5%       5%   50%                                                             Starch                                                                            30%  Sodium                                                                             Sodium                                                                             12.5%                                                                              25%                                                   Dry Starch                                                                             Silicate                                                                           Silicate                                                                           Lignin                                                                             Lignin                                 Binder     Water                                                                             Mixed                                                                             Solution                                                                           Solution                                                                           Solution                                                                           Solution                                                                           Solution                               __________________________________________________________________________    Bulk Density (lb/ft.sup.3)                                                    Green Pellets                                                                            50.7                                                                              41  44.8 46.8 49.7 47.8 52.5                                   Liquid (wt %)                                                                 Green Pellets                                                                            8   8   9.5  8.5  8    8    9                                      Air Dried Pellets                                                                        4.0 5.1 3.5  4.0  3    3    3.8                                    Drop Test                                                                     Green Pellets                                                                            6   3   6    2    2    6    7                                      Air Dried Pellets                                                                        12  4   24+  2    10   12   24+                                    Oven Dried Pellets                                                                       1   3   24+  1    18   3    24+                                    Crush Test                                                                    Green Pellets                                                                            1   1   2    1    3    1    4                                      Air Dried Pellets                                                                        1   1   20   1    11   6    20                                     Oven Dried Pellets                                                                       5.5 1   3    1    2    2    13                                     __________________________________________________________________________

What is claimed is:
 1. A process for stabilizing finely divided residualsilicon powder contaminated with chlorine, hydrogen and oxygencomprising:(a) pelletizing the silicon powder and (b) impregnating thepellets with an organic binder whereby the impregnated pellets ofresidual silicon are made stable for safe transportation and disposal.2. The process of claim 1 wherein the silicon powder is spent contactmass from the production of organchlorosilanes by the direct process andis reactive and exothermic upon exposure to moisture.
 3. The process ofclaim 1 wherein said pelletizing and impregnating are performedsimultaneously.
 4. The process of claim 1 wherein said organic binder is100% liquid organic binder.
 5. The process of claim 1 wherein saidorganic binder is in solution or emulsion.
 6. The process of claim 5wherein the solvent or emulsion carrier is water.
 7. The process ofclaim 6 wherein sufficient water is present during pelletization andimpregnation to quench the heat of reaction.
 8. The process of claim 5wherein said solution or emulsion is from about 10% to about 75% byweight solids.
 9. The process of claim 1 wherein said binder is lignin.10. The process of claim 1 wherein the pelletization is conducted by apressure compactor.